Rare is a very mis-used word in numismatics, try an
ebay search using that as a keyword, for example.
They are uncommon, but from 1968 to 1979, they can be found in all kinds of grades. They are much more common than the larger denomination coins, for example you rarely see 50c struck on 25c planchets, and so on...
Two months ago, ASYN on
ebay sold one graded MS-64 of the most common date (1979), in a PCGS holder, for about $300 (PCGS grading of errors is a $75 cost/investment, which has to be factored in).